Hitherto, as technology for determining the connection phase of a transformer, there has been a proposal for a phase distinguishing device that acquires measurement values of power breakers and sensor-containing partitioning switches through breaker substations, switching substations, and a power distribution monitoring and control master station, and acquires measurement values of smart meters through an automatic meter master station. The phase distinguishing device has power distribution system configuration data, power distribution transformer device data, and consumer supply equipment data stored in a data storage section. A time segment is selected that is a period, out of each of the measurement periods of measurement values, for employment in distinguishing the connection phase. The connection phase of each of the power distribution convertors is then distinguished based on each of the measurement values measured in the selected time segment, and on each of the items of data stored in the data storage section.